Nuclear fusion by inertial confinement, Inertial Confinement Fusion (“ICF”), utilizes nuclear fusion reactions to produce energy. In most types of ICF systems, an external drive mechanism, such as a laser, delivers energy to a target containing nuclear fusion fuel. The target is designed to use this energy to compress, heat and ignite the fusion fuel within the target. If a sufficient amount of fuel is compressed sufficiently and heated sufficiently, a self-sustaining fusion reaction can occur in which energy produced by fusion reactions continues to heat the fuel. This is generally referred to as “ignition.” The inertia of the compressed fuel can keep it from expanding long enough for significant energy to be produced before expansion of the fuel and the resultant cooling terminates the fusion reaction. Most conventional ICF target designs involve a spherical target which is imploded symmetrically from all directions, relying on the stagnation of the inwardly-accelerated fuel at the center of the sphere to produce the required densities and temperatures.
Production of the very high temperatures and densities required for fusion ignition may require a substantial amount of energy. The exact amount of energy required depends on the specific target design in use. In order to be useful for energy generation, the target must be capable of producing more energy from fusion reactions than was required to ignite it. In addition, the amount of energy required by the target must be physically and/or economically realizable by the drive mechanism being used.
For this reason, conventional ICF target designs have focused on achieving the required temperatures and densities as efficiently as possible. These designs are often complex in their construction and operation. They are also sensitive to imperfections in the target's manufacturing, as well as any non-uniformity in the delivery of energy to the target from the drive mechanism. Imperfection and non-uniformity can lead to asymmetry in the target's implosion, which may potentially reduce the densities and temperatures achieved below the threshold required for ignition. Furthermore, successful operation of these complex designs often requires achieving a precise balance between multiple competing physical processes, many of which are poorly understood and difficult to model. When actually constructed and deployed, these complex ICF target designs often fail to perform as their designers intended, and to date none have actually succeeded in producing ignition or the desired fuel conditions.
The National Ignition Facility (“NIF”) target exemplifies the conventional approach. The NIF target involves an outer ablator shell comprising primarily plastic or beryllium with varius dopants surrounding a shell of cryogenic D-T ice with a central void filled with low-density D-T gas. The NIF target is placed in a cylindrical hohlraum. In operation, a laser having of 192 separate beamlines, with a total energy delivered to the hohlraum of up to 1.8 MJ, illuminates a number of spots on the inner surface of the hohlraum, producing a radiation field which fills the hohlraum. The radiation field ablates the ablator layer, and the reactive force of the ablation implodes the target. The laser pulse is 18 nanoseconds long and is temporally tailored in order to drive a series of precisely-adjusted shocks into the target. The timing and energy level of these shocks are adjusted in order to achieve a quasi-isentropic, efficient implosion and compression of the shell of D-T fuel. Stagnation of these shocks and inward-moving material at the center of the target is intended to result in the formation of a small “hotspot” of fuel, at a temperature of roughly 10 keV and a pr of approximately 0.3 grams/cm2, surrounded by a much larger mass of relatively cold D-T fuel. It is intended that the fuel in the “hotspot” will ignite with a fusion burn propagating into the cold outer shell.
At the time of this disclosure, the NIF target has failed to ignite, achieving peak temperatures and densities of about 3 keV and a pr of approximately 0.1 grams/cm2 in the hotspot, which is well short of the 10 keV and 0.3 grams/cm2 that is believed to be required for ignition. There is no clear consensus on what has caused the failure of the NIF target to achieve ignition, but it appears that this failure may be partially due to low-order asymmetry in the hotspot formation and lower than expected implosion velocities.